One potential source of biofuels is to use algae to generate molecules that are suitable for use as fuels. For example, algae, like plants, can generate lipid molecules. Some lipid molecules have a general structure and molecular weight suitable for making diesel additives or can be converted or used as is in diesel fuel, such as fatty acid methyl esters (FAME). It may also be possible to refine certain algae lipids into conventional fuels or fuel blending stocks including gasoline, diesel, and jet fuel. However, many challenges remain in developing commercial scale production techniques for biofuels based on algae production.
Although hybrid systems exist, photosynthetic algae are typically grown in one of two environments—in an open pond environment, or in a closed system such as a photobiorector. One advantage of an open pond is cost, as an open pond can be constructed easily and is relatively inexpensive to operate. Open ponds typically use gravity and/or a paddle wheel to move and mix algae growing within a circular or raceway pond. Open ponds can be made quite large, with sizes of an acre or even several acres being possible for a single pond. An open pond environment also provides high algae growth rates, as a pond can support growth rates that can allow some types of algae to double in number and/or biomass in less than 24 hours.
Unfortunately, open ponds are typically subject to the ambient weather (e.g., temperature, rainfall) and other conditions of the environment surrounding the pond. For example, an open pond is subject to contamination from outside organisms that may compete with or consume the algae being grown. Gases introduced to enhance growth rates in the pond, such as CO2, can be partially lost through the open surface of the pond if the gases are not captured by the algae with sufficient efficiency. Another potential disadvantage of an open pond is water exchange with the surrounding environment. Many ponds have a large surface area and/or a relatively large ratio of surface area to volume. The large ratio of surface area to volume is typically due to the ponds having a relatively shallow depth, typically about 15 cm to about 75 cm. Because evaporation is proportional to the exposed surface area of a body of water, evaporation of water from the pond can require addition of substantial make-up water in order to maintain a constant concentration. An open pond is also subject to dilution by excessive rain.
Still other potential difficulties with open ponds are related to the harvesting of desired products. Any product secreted by the algae in a pond may evaporate to an extent, or be consumed by bacteria and/or other organisms, resulting in loss of product. For this reason, algae are conventionally first grown and then processed in a series of steps that extract products (e.g. oils, proteins). Concentration and processing of the algae can be complex, as algae typically grow at concentrations in water of less than about 1000 ppm and, more commonly, about 200 ppm to about 400 ppm.
A closed photobioreactor (PBR) system can mitigate several of the problems described above. A PBR system is typically sealed to reduce or prevent contamination from environmental sources. Gasses that evolve from the algae, and gases introduced to grow the algae, can also be managed to a greater extent. Additionally, the algae can be mixed to an extent that optimizes average solar exposure. Water evaporation or dilution can also be controlled or eliminated. Typically, closed photobioreactors enhance the rate of algae growth by 50-100% when compared to open pond systems. However, closed photobioreactor systems are typically much smaller than open ponds, with many photobioreactors (i.e., typically hundreds to thousands) needed per acre of land to achieve the same productivity. Further, relative to a pond, the construction and operation of closed photobioreactor systems is expensive and complicated. Additionally, even during ideal operation, the algae are still not concentrated to greater than a few 1000 s of ppm—although values of up to 1% and above have been reported.